In the early days of the telephone art, use of the telephone was often confined to communications among users within a local geographic area. As a result and over the years, the economies related to accessing a communications system have lead to telephones in a local area usually being interconnected through a central controller, often called a local central office in the art.
As digital computers came upon the scene, another local community of use became discernible. Hence, a central controller is commonly employed for interconnecting various user terminals. For example, U.S. Pat. No. 3,851,104; entitled "Digital Communications System" and issued Nov. 26, 1974; discloses a time division, multiple access communication system which, under the control of a central terminal, provides communication links among a plurality of user terminals by way of a single communications signal path.
As the digital computer art advanced, parallel advances in the semiconductor art have lead to smaller, relatively inexpensive computers. With the advent of such smaller computers, the technique of central computer control is being abandoned in favor of a distributed control technique. Also, because of the usually bursty nature of digital information, the recent trend has also been toward communications systems having a capability for handling packets of digital information. One such communications system, which may be of a type commonly referred to as a local area network, is disclosed in a copending application by C. F. Flores et al, i.e., Ser. No. 399,428, entitled "Collision Avoiding System and Protocol for a Two Path Multiple Access Digital Communications System", and filed July 19, 1982, which application is herein incorported by reference and which application discloses a collision avoiding communications system which system contemplates a communications loop having first and second oppositely directed unidirectional communications signal paths. Each of at least two stations such as a digital computer may be coupled to both the first and the second paths. A station may write a first packet signal on the first path and read a second packet signal from the second path. A station may also read a third packet signal from the first path. The arrangement for reading the third signal may be electrically preceding the arrangement for writing the first signal on the first path. A station may also write a fourth packet signal on the second path. The arrangement for writing the fourth signal may be electrically succeeding the arrangement for reading the second signal on the second path. In response to the first, second, third and fourth signals, the communications system through its stations and protocol avoids collisions on the communications paths. Its protocol contemplates that a busy field of a packet may indicate that a respective path is either busy or not busy. If the path is not busy, the station may write a packet on the path. If the path is busy, the station may overwrite the busy control field for indicating that the station has a packet to transmit and that it is contending for the path.
Of concern here is the design of a local area network for the transmission of mixtures of various traffic types such as, for example, mixtures of voice traffic, video traffic and facsimile traffic as well as digital computer data traffic. Clearly, it is desirable to permit different types of traffic to be efficiently combined on one system. This presents a challenge because the different types of traffic have different characteristics.